There has been considerable interest in recent years in nonaqueous secondary cells because of the possibilities afforded of obtaining cells useful for small electronic applications, such as calculators, watches and circuit boards, which overcome some of the drawbacks, e.g., self-discharge and low voltage, of the presently used nickel cadmium cells. Many materials have been considered as candidates for the electrode materials in such cells.
Particular attention has been given to those classes of materials that undergo topochemical reactions because of their potential utility as the positive electrode material in secondary cells. Broadly speaking, topochemical reactions involve a host lattice into which a guest species is inserted with the product of the host and guest maintaining the essential structural features of the host. If the structural changes are minimal, the process is termed intercalation and the overall reaction is likely to be readily reversible. If the reaction is of a redox type, the reversibility of the intercalation reaction may form the basis of a secondary cell.
Several classes of host materials have been identified as candidates for the positive electrode material in room temperature cells using an alkali metal, e.g., lithium or sodium, as the guest species and negative electrode material. One such class is formed by Van der Waal's bonded layered transition metal dichalcogenides in which the structural changes between MX.sub.2 and LiMX.sub.2 ; M=Group IVB, VB, VIB, X=S or Se; involve only a small separation of the MX.sub.2 layers to accommodate the lithium or sodium atoms.
However, compounds containing sulfur or selenium are less desirable, other features being identical, than are compounds containing oxygen for at least two reasons. First, oxides weigh less than the sulfides and selenides and may permit higher energy densities both per unit weight and per unit volume. Second, oxides are typically less noxious and consequently are easier to work with than are sulfides.
Several metal oxides have been investigated as positive electrode materials in cells. For example, MnO.sub.2 was studied by Ikeda et. al., Manganese Dioxide Symposium Proceedings, 1, 384, (1975); WO.sub.x, where x is between 2.0 and 2.9, was studied by Kamenski, U.S. Pat. No. 3,873,369; and V.sub.2 O.sub.5 was studied by Dey and Sullivan, U.S. Pat. No. 3,655,585.
While these studies indicate that metal oxides are promising candidates for positive electrode materials in nonaqueous cells, they are limited in scope. The materials were studied only as candidates for positive electrode materials in primary cells, and the studies did not recognize the importance of both the crystal and chemical structure of the electrode material for the construction of secondary cells.